Most radar systems, especially those that have lower probability of interception (LPI), operate at limited average transmit powers. LPI systems also may involve wideband transmit waveforms (spread spectrum) instead of single-frequency waveforms. In order to increase the detection range of these radar systems, transmit pulses of longer duration, or even continuous (cw) waveforms, are often used. However, the range resolution is reduced as the pulse is lengthened. Pulse compression techniques are available that sub-divide the pulse into a number of shorter intervals in which the waveform frequency or phase is coded in a way that makes those intervals distinct. The radar return waveform is processed in such a way that the various intervals are overlaid in time to create a much shorter effective pulse of higher energy. For example, many radar systems employ transmit pulses that have a duration of 10 to 500 microseconds. In comparison, the pulse needs to be compressed to approximately 2 nanoseconds to achieve a range resolution of 1 foot. Pulse compression ratio is defined as the ratio of the transmit pulse duration and the sub-divided pulse interval. Thus, there is a desire to achieve large pulse compression ratios since that improves the processing gain of the radar system.
Phase coding is one way to achieve large pulse-compression ratios and is used in many radar systems. Presently, phase coding has only been used for narrow-band radar systems, partly because of the difficulty of generating and processing wideband waveforms by electronic means.
The RF-lightwave approach disclosed herein is compatible with wideband uncompressed waveforms that may be useful for LPI systems. In fact, this approach can be used with a variety of waveforms. The disclosed approach also can potentially achieve shorter sub-divided pulse intervals, which could lead to larger pulse compression ratios and finer range resolutions or improved processing gain. Because the short sub-divided pulse interval can be achieved, the approach disclosed herein also can be used to compress, by phase coding, individual pulses in the pulse bursts that often are employed in radar systems. Bursts of short pulses have high pulse-repetition frequencies, with each burst separated by longer intervals. This can reduce the range and Doppler ambiguities.
The disclosed approach preferably combines the benefits of large pulse-compression ratios, short compressed pulses and compatibility with a variety of wideband waveforms.
Improved range resolution allows the radar system to not only detect the presence of objects but also to identify them by detecting their features. The disclosed approach makes possible the achievement of pulse compression with wideband LPI waveforms.
The prior art includes electronic methods for pulse compression by phase coding, and a large number of pulse compression phase codes are known as are the radar systems that employ phase-coded pulse compression. The presently disclosed technology makes use of conventional phase codes and likely can also make use of future phase coding techniques as well. Examples of conventional phase codes are discussed in a book chapter on Phase-Coding Techniques by Cohen and Nathanson in Radar Design Principles, 2nd Ed., SciTech, 1999.
Prior art approaches for using phase encoding in radar systems typically involve direct changes of the phase at the microwave carrier frequency. Microwave “magic-tee” transmission line structures provide anti-phase outputs and “hybrids” provide 0 and 90° phase shifts over bandwidths in excess of 20 percent of the carrier frequency. Semiconductor diode switches, which can have switching speeds of a few nanoseconds, are typically used to select the phase. Thus, the sub-divided pulse intervals are at least many nanoseconds in duration. Digital approaches also can be used to generate phase-shifted waveforms. Digital synthesizers, however, are generally limited to frequencies of several hundred megahertz or lower.
The processing of radar return signals is typically done using analog microwave tapped delay lines or by using digital shift registers. The tapped delay lines can operate at the microwave carrier frequency or at a lower, intermediate frequency. Some prior tapped delay lines operate after the return signal has been down-converted to video frequencies. Typically, lengths of microwave cable or transmission lines are used as the delay lines. The tapped delay-line function also can be accomplished by surface acoustic wave (SAW) devices. For each tapped signal, an appropriate phase shift, using the approaches described in the preceding paragraph, is applied to counteract the phase shift produced at the encoder. The outputs from the various re-shifted taps are then summed together. For high-frequency signals, the microwave implementations of the tapped delay line approaches can limit the cumulative delay (the delay increment times number of taps) because of the attenuation of the delay lines. Also, the phase re-shifts generally cannot be changed quickly. Digital techniques typically involve sampling and quantizing the return signal and then moving that sampled data down a shift register. The sampler and shift register can be clocked at the sub-divided interval. The phases of the data samples in the register are then compared with a template pattern to determine a match. Since only the phase or sign of the data samples are compared, the quantizer can be quite coarse in terms of resolution. The fastest digital samplers are capable of clock rates of several gigahertz.
The presently disclosed technology also preferably makes use of tapped delay line paths, similar to some of the decoding architectures. A new way to accomplish delays for time-delay encoding/decoding, by using switched optical delay lines, is disclosed. A key advantage of the photonic approach for encoding described herein is that the subdivided pulse interval can be fractions of a nanosecond long. This leads to improved range resolution. Likewise, the counteracting time-delay shifts (the time-delay re-shifts) applied to the tapped signals in the decoder can be changed quickly—at speeds in excess of several gigahertz. This can allow the decoder to be reconfigured or adapted rapidly to account for effects such as Doppler shifts from closely spaced targets.
Switched optical delay lines have been used for RF antenna beam forming. Tapped optical delay lines have been used for constructing RF filters as well as for beam forming. It is not believed that there exists any prior use of switched or tapped optical delay lines to construct RF time-delay encoders or decoders for pulse compression.
Switched optical delay lines have been used in the past for phase-shift keying of signals for communications applications. These phase modulators are described by Fukushima, Doi, et al., in articles published in J. Lightwave Technology, v. 18, p. 301 (2000) and in IEEE Photonics Technol. Letters, v. 11, p. 1036 (1999). The architecture of these prior phase modulators is somewhat similar to the architecture of the time-delay encoders disclosed herein. For these prior phase modulators, however, a single-frequency microwave signal is impressed on the lightwave carrier. In contrast, the time-delay encoder disclosed herein may be used with both single-frequency and wideband RF waveforms.